Paracetamol (INN of acetaminophen or N-(4-hydroxy-phenyl)acetamide) is an analgesic and an antipyretic widely used in hospitals. It is desirable to have available stable liquid pharmaceutical formulations of this active principle for administration by injection, in particular for intravenous infusion.
It is known that paracetamol in aqueous solution is liable to undergo hydrolysis to form p-aminophenol, which is itself liable to degrade into quinoneimine (cf. for example J. E. Fairbrother, “Acetaminophen” in Analytical Profiles of Drug Substances, 1974, vol. 3, pp. 1-109). The rate of degradation of paracetamol increases with increasing temperature and light. This rate is minimal at a pH in the region of 6 (K. T. Koshy et al., 1961, J. Pharm. Sci. 50, pp. 116-118).
It is known practice to add a buffer and an antioxidant or free-radical scavenger to stabilize paracetamol in solution.
WO 02/072 080, for example, describes stable aqueous paracetamol solutions for infusion comprising a buffer of pH 5.5 to 6.5 and an antioxidant chosen from ascorbic acid and a derivative bearing a thiol function.
EP 0 916 347 discloses-paracetamol solutions based on a mixture of water and of alcoholic solvents comprising a buffer of pH 5.5 to 5.6 and metabisulfite as antioxidant.
EP 0 859 329 describes a deoxygenation process by which the aqueous solvent is deoxygenated by bubbling into an inert gas, such as nitrogen.
Also US 2004/0054012 describes a deoxygenation process involving the bubbling of an inert gas such as nitrogen through the aqueous solution.
WO 2008/135601 describes aqueous paracetamol solutions for infusion prepared using high temperatures and in an oxygen-free environment.
Some of the prior-art stabilized injectable solutions of paracetamol have the drawback of requiring the total absence of oxygen during the production process. However, oxygen shows a very great facility to dissolve in water, making it necessary to ensure that the solution, once deoxygenated, does not subsequently come into contact with atmospheric air at every stage in the production process. The methods of the art, therefore, require a considerable amount of time, care, and the use of specialised equipment and/or protocols. Even with all these precautions, total absence of oxygen can never be ensured during all the preparatory steps. In presence of even traces of oxygen, degradation products can be generated which increase in quantity during storage of the product, and ultimately leads to a reduced shelf-life.
The main object of the present invention is to provide a formulation and a method for aqueous formulations of paracetamol, which can notably be utilized in injectable preparations being stable over a long period, which solves the problems in view of the problems of the art.